I hope you guys don't mind me asking a question as my 1st post... I have been lurking a little on this forum but now coming out of the shadows I'm trying to get my 2000 yz426f ready for the long trip on the weekend, but here comes the new engine noise! that engine is a bit on the loud side indeed but this is noticeably new. its kinda like this http://www.youtube.com/user/twindrag.../2/Dthdc2Q4TnY but not nearly as loud and does _not_ go away with higher RPM. it does however _completely_ go away when engine braking. Things done so far: Gas is fresh, premium, suspected detonation and tried to disconnect tps without any change. No loose engine mounts, including the pivot bolt. cam chain is fairly tight but not ideally timed. It is a hair off, with cam dots ruller-perfect the crank timing mark is 1/8 inch forward of the case mark, and adjusting the cams a tooth off will give worse results. one exhaust valve and two intakes are borderline on the loose side but not out of spec. The comp release lever is not hitting the bucket. The top end is very clean and is getting plenty of oil. i've stumbled on one of the posts on TT from few years back (http://www.thumpertalk.com/forum/archive/index.php/t-585085.html) regarding the primary gear on the crank getting loose, so I'm gonna check that next as I'm one step away from pulling the cover now. or adjust the valves a bit tighter, button it up and try that... it still starts fine and runs strong w/o any power problems. Oil has been changed religiously every other ride since i got the bike 3 months ago. It is my 1st time experience with these bikes (or any dirt 4stroke) so im curious: Does this sound like a loose primary gear? Do they like to run a little tighter on the valves or a little looser? (I always thought that slightly on the loose side of the valve clearance limits is better) How right should the cam chain be? The bike was fine before last weekend's ride and after half a day at the track and few trails the offending sound appeared. today i will get a boroscope from work and peek inside the cylinder and cam chain cavity... thanks a bunch for your time reading this and any advice! ordering some gaskets and spare primary gear key now... i need to get this beast running quick to join a friend this weekend!